1. Field of the Invention
The present invention generally relates to a panel light device, and more particularly, to a back light module for a liquid crystal display (LCD).
2. Description of the Related Art
Referring to FIG. 1, it depicts the structure of a conventional liquid crystal display. Generally, the liquid crystal display device includes a liquid crystal panel 10 which has two substrates and a liquid crystal material sealed therebetween, a back light unit 20 disposed under the liquid crystal panel 10, and cases 11 and 12.
The back light unit 20 is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel 10. There are several kinds of back light units 20 such as a direct back light type (or direct type) and an edge light type.
Referring to FIG. 2, which is a cross-sectional view along line 2—2 of FIG. 1, it depicts a back light module 21 of the direct type. The direct type back light module 21 includes a housing 70 which has a reflective sheet 60 disposed on the bottom surface of the housing 70 and, a lamp 50, such as a fluorescence cathode tube, disposed at the bottom portion of the housing 70, a diffusing sheet 40 disposed on the upper surface of the housing 70 and a prism sheet 30 disposed on the diffusing sheet 40. Since the lamp 50 is spacedly disposed inside the display area of the liquid crystal panel 10, the brightness distribution of the liquid crystal panel may be not uniform. Also, the TFT (Thin Film Transistor) switching elements of the liquid crystal panel 10 may be damaged by the heat of the back light source.
Therefore, some distance or space between the lamp 50 and the diffusing sheet 40 is required. This distance or space is provided typically by means of supports 55 disposed in the housing 70. Because of these required structural elements, it is very difficult to reduce the thickness of the direct type back light module 21.
Referring to FIG. 3, which is a cross-sectional view along line 2—2 of FIG. 1, it depicts a back light module 22 of the edge light type. The edge light type back light module 22 includes a light guide 80, a lamp 50 which is attached to at least one edge of the light guide 80, and a U-shaped reflector 61 which surrounds the lamp 50. An open portion of the reflector 61 is disposed at the edge of the light guide 80, a reflecting sheet 60 is disposed at the bottom of the light guide 80, a diffusing sheet 40 is disposed on the light guide 80 and a prism sheet 30 is disposed on the diffusing sheet 40. Because the lamp 50 is disposed at the edge of the light guide 80, the thickness of the LCD can be relatively decreased.
The light guide 80 includes a printed dot pattern or a V-shaped notched pattern on at least one surface for scattering the light in the light guide 80 and illuminating the liquid crystal panel 10. The light guide 80 is typically made of PMMA by the process of press or ejection molding. The diffusing sheet 40 is disposed on the light guide 80 and typically made of half-transparent PET or polycarbonate for further evenly diffusing the light emitted from the light guide 80. The prism sheet 30 is disposed on the diffusing sheet 40 for gathering the diffused light from the diffusing sheet 40 in the direction perpendicular thereto.
Since the liquid crystal panel 10 has a polarizing sheet for transmitting the light with one polarizing direction and absorbing the light with the other polarizing direction, about 50% of the energy of the light is lost when the light passes the polarizing sheet. Therefore, a polarization recycle film 35 is typically disposed on the prism sheet 30 for reflecting the light with the other polarizing direction, and the reflected light then is reflected by the optical element therebeneath and recycled after the polarizing direction thereof is changed, thereby increasing the brightness of the liquid crystal display. However, the above-mentioned polarization recycle film is available typically from 3M™ Company of St. Paul Minn. under the trade name Dual Brightness Enhancement Film (DBEF), which causes the liquid crystal display up to 160-170% brighter, but is significantly expansive. Further, the cost of the liquid crystal display will increase more and more as the dimension of the liquid crystal display increases in recent years.
Accordingly, the back light module of the liquid crystal display shall be constructed to meet the requirements of increasing power efficiency and the screen brightness, providing uniform brightness, lowering power consumption and cost, as well as decreasing the dimension. Prior art attempts have been made to meet the requirements and, for example, can be seen in U.S. Pat. No. 6,164,790 issued to Lee on Dec. 26, 2000, U.S. Pat. No. 5,477,422 issued to Hooker et al. on Dec. 19, 1995, and U.S. Pat. No. 5,485,354 issued to Ciupke et al. on Jan. 16, 1996. These patents are all incorporated herein by reference. However, the back light modules in these patents and the other prior art still cannot meet the above-mentioned requirements.
Furthermore, U.S. Pat. No. 6,086,212 issued to Onishi et al. on Jul. 11, 2000 discloses a back light unit which comprises an output light modulator provided on the light guide and an input light modulator provided between the light source and the light guide. However, the structure of the back light unit is complex and the brightness of the liquid crystal panel still cannot be significantly increased.
Accordingly, there exists a need for a back light module of a liquid crystal display capable of meeting the above-mentioned requirements.